AC input voltage interruption detection circuits are used for detecting interruption of AC input voltage, and for performing, for instance, shut off of a power supply that is connected with AC input voltage or discharge of capacitor that has been brought to high voltage, and are essential circuits for the safe operation of AC-DC converters and the like.
FIG. 1 and FIG. 2 are diagrams for explaining a conventional first AC input voltage interruption detection circuit, and the operation thereof, as disclosed in Japanese Patent Application Publication No. 2009-165305 (FIG. 1 to FIG. 3). In FIG. 1, the voltage of a filter capacitor C0 follows AC voltage when AC input voltage is connected via a plug that is connected to an AC power supply. However, when the AC input voltage is interrupted for any reason, the AC voltage at that point in time is held at C0, after which the AC voltage is discharged gradually by way of resistors (R1 and R2, and R3 or R4). The AC input voltage is monitored by a detection circuit (detailed circuit illustrated on the right of FIG. 1) via a divided voltage Vin from resistors R1 and R2 by way of a connection point of these resistors. As illustrated on the right end of FIG. 2, AC is determined to be interrupted if a state where Vin does not drop to or below a reference voltage Vref1 lasts for a predefined time that is longer than the AC period.
In a more detailed explanation, the detailed circuit illustrated on the right of FIG. 1 has: a first comparator CMP1 having a hysteresis characteristic and having the input voltage Vin applied to an inverting input terminal thereof; a switch MOSFET Q1, a gate terminal whereof is connected to an output terminal of the comparator CMP1; a constant current source I1 connected between a MOSFET Q1 and a power supply voltage terminal VDD; and a capacitor C1 connected between a ground point and a join node N1 of Q1 and I1. The constant current source I1 charges the capacitor C1 at constant current in the period during which Q1 is off. As a result, when a potential V1 at the node N1 rises gradually and Q1 is switched on, the charge of the capacitor C1 is discharged, whereby the potential V1 is lowered rapidly, to generate thereby a saw-tooth signal.
The detailed circuit illustrated on the right of FIG. 1 also has: a second comparator CMP2 having a hysteresis characteristic and having the potential V1 applied to a non-inverting input terminal thereof; an open-drain output MOSFET Q2, a gate terminal whereof is connected an output terminal of the comparator CMP2; and a constant current source I2, a diode D3, and dividing resistors R5, R6, that generate the reference voltage Vref1 that is applied to the non-inverting input terminal of the first comparator CMP1 and a reference voltage Vref2 that is applied to the inverting input terminal of the second comparator CMP2; wherein the drain terminal of Q2 is connected to an output terminal OUT.
The first comparator CMP1 operates by comparing the input voltage Vin and the reference voltage Vref1, such that when the input voltage Vin becomes higher than Vref1, the output of the comparator falls to a low level, and the MOSFET Q1 is switched off, and when Vin becomes lower than Vref1, the output of the comparator rises to a high level, and Q1 is switched on. When Q1 is switched on, the charge of the capacitor C1 that is connected to the join node N1 of Q1 and I1 is withdrawn, and, as a result, the potential V1 of the node N1 changes to the ground potential. The second comparator CMP2 operates by comparing the potential V1 of the node N1 with the reference voltage Vref2, such that when V1 becomes higher than Vref2, the output of the comparator falls to a low level.
In a further explanation of the operation of the above circuit configuration, the first comparator CMP1 detects an AC waveform and continuously outputs a pulse even if AC voltage drops somewhat, as in period T1 illustrated in FIG. 2. Therefore, the potential V1 of the node N1 does not become higher than Vref2 through periodic resetting of the charge of the capacitor C1, and an output Vout remains at a high level. When, by contrast, the AC waveform disappears, as in period T2 illustrated in FIG. 2, the first comparator CMP1 outputs no more pulses, and the charge of the capacitor C1 is no longer reset. Accordingly, V1 becomes higher than Vref2, the output Vout changes to a low level, and interruption of the AC power supply (AC input voltage interruption) can be notified to the exterior.
FIG. 3 and FIG. 4 are diagrams for explaining a conventional second AC input voltage interruption detection circuit, and the operation thereof, as disclosed in Japanese Patent Application Publication No. 2009-89490 (FIG. 2 and FIG. 4). In the circuit shown in FIG. 3, unlike in the case of the conventional first AC input voltage interruption detection circuit illustrated in FIG. 1, a ripple component of detected voltage is extracted by an AC detection unit (high-pass filter), and it is determined that an and AC input voltage is connected if the voltage resulting from rectifying and smoothing the extracted ripple voltage exceeds a reference voltage Vr3.
In a more detailed explanation, a differential amplifier 22 of the circuit of FIG. 3 works out, by way of an output voltage detection unit 11, a difference value of voltage-to-ground detection signals of output power supply lines PL1, PL2; a ripple component is detected by an AC detection unit 23; the ripple component is rectified and smoothed by a rectifying and smoothing unit 24; and a comparing unit 25 compares the result versus the reference voltage Vr3. When a short-circuit fault occurs in a capacitor C3, however, the line voltage between the output power supply lines PL1, PL2 becomes substantially zero, and the ripple component generated between the lines becomes likewise almost zero. As a result, the output signal level of the rectifying and smoothing unit 24 takes on a value of zero or close to zero, and a low-level alarm signal of occurrence of a capacitor short-circuit fault is outputted by the comparing unit 25.
FIG. 4 is a waveform diagram illustrating the details of capacitor short-circuit fault detection of FIG. 3. If capacitors C1 to C3 are in a normal condition, a DC voltage that comprises a switching frequency ripple component by switching elements Q1 to Q4 is applied to the output power supply lines PL1, PL2, and the detection signal by the output voltage detection unit 11 takes on a DC voltage that comprises the ripple component, as illustrated in (A), but becomes almost 0V when the capacitor C3 is shorted. The AC detection unit 23 detects the voltage comprised in the ripple component, as illustrated in (B), such that the detection signal becomes 0V upon shorting of the capacitor C3. As a result, the rectifying and smoothing output signal of the rectifying and smoothing unit 24 becomes 0V when the capacitor C3 is shorted, as illustrated in (C) and (D). The output signal of the comparing unit 25 that compares the rectifying and smoothing output signal with the reference voltage Vr3 is of a high level, as illustrated in (E), if the capacitor C3 is in a normal condition, but takes on a low level of 0V as a result of the occurrence of a short-circuit fault in the capacitor C3. This low-level signal is extracted as an alarm signal of occurrence of a capacitor short-circuit fault.
In the conventional second AC detection circuit scheme described above, the occurrence of the capacitor short-circuit fault is detected through extraction of a ripple component alone. As a result, the below-described problem does not occur, as does in the conventional first AC input voltage interruption detection circuit illustrated in FIG. 1, where the detected voltage is compared with a fixed level.
In the above conventional first AC input voltage interruption detection circuit, the input voltage Vin may in some instances fail to drop down to the reference voltage Vref1, even if, when a time constant, which is based on a capacitive component (including a parasitic capacitance as well as a deliberately added capacitance for surge protection) that is present further toward the detection circuit side than the diodes D1, D2 and which is also based on the resistors (R1, R2) that discharge the capacitive component, is large. In such a case, a problem arises where erroneous determination is made that the AC input voltage is interrupted. If the circuit is used with fixed AC voltage, this problem can be avoided by selecting a reference voltage Vref1 corresponding to a voltage that is slightly lower than the peak value of the AC input voltage, but a problem arises then in that the choice of the reference voltage Vref1 becomes difficult when the AC input voltage range is expanded in worldwide-compliant power-supply specifications.
The above-described conventional second AC input voltage interruption detection circuit does not suffer from the problem of such erroneous determination, as is the case in the conventional first AC input voltage interruption detection circuit, but requires a large resistance and/or capacitance for filtering and rectifying low-frequency components, from about 50 Hz to 60 Hz, and is therefore problematic in that the AC input voltage interruption detection circuit is not suitable for being configured in the form of an integrated circuit.